The present invention relates to a coated electrode which is obtained by coating an alloy composed of lead and silver with lead dioxide (hereinafter abbreviated as PbO.sub.2).
Cathodes used for the electrolytic refining, electrolysis of salt water, and electrolytic processing of industrial waste water, must have a good electric conductivity as well as an excellent resistance against corrosion. The electrode which is composed of an electrically conductive material such as metal coated with a thin layer of PbO.sub.2, satisfies the above-mentioned requirements and features reduced weight. The ordinary metals, however, easily develop a film of oxide on the contacting surface which contacts to the layer of PbO.sub.2. Further, since the contact resistance is remarkably great between the metal and PbO.sub.2, it becomes difficult to flow an electric current. Therefore, a particular attempt has been made to impart a high electric conductivity to a coated-type lead dioxide electrode which has a metallic substrate. For example, Japanese Patent Publication No. 23947/76 discloses an electrode obtained by electrodepositing PbO.sub.2 on the surface of a silver substrate or a metal substrate plated with silver. This electrode has a very small contact resistance between the metal substrate and the PbO.sub.2 coating. That is, it is possible to directly flow a current from the metal substrate which exhibits good electric conductivity. Further, the electrode exhibits a sufficient resistance against corrosion when it is used for the electrolytes containing halogen, sulfides, chromates and carbonates that form sparingly soluble silver compounds, or for the alkaline electrolytes.
When the electrode is used in the electrolytes other than the above-mentioned electrolytes for extended periods of time, however, it often becomes difficult to flow the electric current from the substrate being caused by the fact that silver elutes out at a small rate onto the PbO.sub.2 layer through pores that reach the silver surface, and that there exists a gap between the substrate and the PbO.sub.2 layer.
Another defect of the coated electrode employing silver substrate or silver-plated substrate is that when PbO.sub.2 is to be electrodeposited on the silver substrate, it is allowed to use only the alkaline electrolyte to prevent silver from being anodically dissolved, and that when the silver-plated substrate is to be used, it is required to increase the thickness of the plated layer to increase the electric conductivity, requiring increased cost.
It has been attempted to electrodeposit the PbO.sub.2 layer on the substrate made of pure lead. In this case, however, it is observed that the PbO.sub.2 layer peels off from the substrate during the step of electrodeposition or when the coated electrode is being used. The cause is attributed to that heat is generated due to a very large electric resistance at the contact surface between the lead substrate and the PbO.sub.2 layer, and stress is generated due to difference in the thermal expansion such that the layer is peeled off from the substrate, and that the electrolyte infiltrates through the clearance to corrode the lead substrate.